The present invention relates to a resonance mode converter power supply, and more particularly, to a controller adopting a zero voltage switching system and an electronic ballast using the same.
In general, there are two kinds of power supplies. One is a series power supply for continuously feeding power and the other is a switching mode power supply (SMPS) having enhanced efficiency and reduced volume by employing a switching element. Recently, the demand for the SMPS-type power supply has increased greatly and the switching frequency thereof has gradually increased as well, in accordance with the continuing trend towards the manufacture of electronic appliances which are lightweight, thin, simple and small electronic appliances. As one kind of SMPS, the newly introduced resonance mode power supply (RMPS), more specifically, a resonance mode converter, is known for its high efficiency, reduced volume and good EMI characteristics. In the converter employing such a switching system, the switching frequency is increased to decrease the number of components, increase electric power efficiency or decrease the ripple of the output. This, however, unfortunately results in an increase of switching electric power loss. That is, in a power supply of the conventional apparatus, the switching element is constituted by a power semiconductor device and is generally a bipolar transistor or field effect transistor (FET). Accordingly, high switching frequencies tend to decrease power efficiency and cause greater stress on the switching element.
FIG. 1A is a schematic view showing the conventional converter which has an input power Vd supplied to an input, a switch SW1 connected in series with the positive input terminal, a diode D.sub.A1 reverse-bias connected between the output side of switch SW1 and the negative input terminal, a capacitor C1 connected in parallel to a load, and an inductor L1 connected in line between the capacitor-load combination and the cathode of diode D.sub.A1. Thus, power is provided to the load according to the operation (opening and closing) of switch SW1. Here, Vds is the voltage developed across switch SW1, and id is the current through switch SW1.
FIG. 1B is a waveform chart showing voltage Vds and current id which are developed across and flow through switch SW1, respectively, as the device of FIG. 1A operates. In addition as shown in FIGS. 1B and 1C, SW' of FIG. 1C represents the timing when switch SW1 is turned on (high) and off (low), id is the waveform of current id flowing through switch SW1 of FIG. 1A, Vds' is the waveform of voltage Vds developed across switch SW1, "a" is the power loss when switch SW1 if turned off, and "b" is the power loss when switch SW1 is turned on.
As shown by FIGS. 1A, 1B and 1C, when switch SW1 is turned on, the current id starts to flow through the switch, and then, even when switch SW1 is turned off, current id does not immediately fall to zero and a current value relevant to the amount of region "a" continues flowing. Likewise, voltage Vds is developed across switch SW1 and is maintained at a steady state with switch SW1 open. Voltage Vds does not immediately fall to zero even when switch SW1 is turned on, and a voltage relevant to the amount of region "b" remains. Accordingly, when switch SW1 is turned off, electric power relevant to the amount of region "a" is wasted, and when switch SW1 is turned on, electric power relevant to the amount of region "b" is consumed as heat in switch SW1. As for such electric power loss caused by the on/off operation of switch SW1, the ratio of the electric power loss to the whole period increases when the switching frequency is increased so as to increase the ripple of the output voltage or to reduce the inductance and capacitance used in the converter. Thus, the overall system efficiency is lowered. That is, the conventional control apparatus increases switching stress and power loss when the switching frequency is increased.
FIG. 2 is a schematic view showing the conventional electronic ballast which includes first and second switching elements Q.sub.11 and Q.sub.12 across which a direct-current (DC) voltage Vdd is generated by rectifying an alternating-current (AC) input Vin, a transformer C.T having a primary winding n11 serially connected to an inductor L.sub.10 and a capacitor C.sub.12 and two secondary windings n12 and n12' one terminal of each being connected to the gates of first and second switching elements Q11 and Q12 via resistors R.sub.A, R.sub.B, R.sub.C and R.sub.D, respectively, a lamp connected in parallel with capacitor C.sub.12, capacitors C.sub.11 and C.sub.13 connected between one terminal of capacitor C.sub.12 and first and second switching elements Q.sub.11 and Q.sub.12, and diodes D.sub.11 and D.sub.12. Thus, when switch SW2 is turned on, the gate of first switching element Q.sub.11 is triggered via a resistor R.sub.1 ' and a capacitor C.sub.1 '.
At the moment when switch SW2 is turned on, and thus first switching element Q.sub.11 is also turned on, a lamp driving current flows through capacitors C.sub.11 and C.sub.12, inductor L.sub.10 and primary winding n11. When capacitor C.sub.11 completes charging, a reverse electric power is generated across secondary winding n12'. Thus, a driving current flows through primary winding n11, inductor L.sub.10 and capacitors C.sub.12 and C.sub.13 as second switching element Q.sub.12 is turned on. Here, when capacitor C.sub.13 completes charging, a reverse electric power is generated across secondary winding n12. Thus, first switching element Q.sub.11 is turned on again. When a switching frequency which repeatedly turns on and off first and second switching elements Q.sub.11 and Q.sub.12 matches the resonant frequency of a series resonant circuit created by inductor L.sub.10 and capacitor C.sub.12, a high voltage is generated across capacitor C.sub.12, which lights the lamp.
The conventional device not only has no function for enhancing the lamp's durability, it in fact accelerates the aging process of the lamp. Moreover, most of the conventional devices employ a hard switching system, which causes an increase of switching loss and switching element damage due to overheating. Thus, system stability cannot be assured and noise is generated.